Legal Question in Real Estate Law in California

You should check with a physician, but I do not believe she can transmit her medical problem to you unless you use the same needle, etc. You do not say whether you had more than a month to month agreement, but she can not change the terms of the lease without giving you 30 days notice. If she did it timely, then you agreed to it but under false information so you could argue the lease is breached and you can move out. You probably are not going to get her out.

How did the landlady "force" you to sign the agreement allowing her to share your place? If she used actual force or threat of force, like twisting your arm or pointing a gun at you, then the agreement is unenforceable and she cannot lawfully come and go and sleep is your place. If however there were only some non-violent threat such as termination of the oral rental agreement, then the space-sharing agreement is probably enforceable as a modification of your original oral agreement.

In your circumstances, I would move out, probably immediately, or after using up any deposit I'd made, and if sued in small claims, I'd tell the whole story as my defense.

In the future, get your rental agreements in writing and don't accept any change to the deal that you can't live with afterwards.